The invention relates to a process for carrying out a non-contact remote interrogation in a system comprising a group of mobile transponders, wherein an interrogation station emits an interrogation signal, said interrogation signal is converted into an information-carrying response signal in a transponder comprising a SAW element and then returned to the interrogation station 1.
The invention furthermore relates to an arrangement for carrying out the process, a transponder for such an arrangement, a SAW element suitable for a transponder and an algorithm for the reduction of disturbing influences on the propagation delay of the response signal.
Processes of the above-mentioned kind which describe the identification of transponders are known e.g. from U.S. Pat. Nos. 4,737,790 (Skeie et al., X-Cyte Inc.), 4,734,698 (Nysen et al., X-Cyte Inc.) and 4,096,477 (Epstein et al., Northwestern University). In these processes passive SAW transponders (so-called SAW tags) are identified by means of an interrogation station. The transponders comprise a suitably packaged SAW element (SAW=surface acoustic wave) consisting of a piezoelectric material and suitable antennas for receiving and emitting electromagnetic waves in the range of 905-925 MHz. The SAW element modifies the received interrogation signal and generates a plurality of response signals each having a characteristic propagation. Two different processes are used for encoding the SAW element. The U.S. Pat. No. 4,096,477 uses a SAW element which does not comprise reflectors and on which a binary encoding is realized due to the use or omission of an output transducer. The response signal thus contains a different, code-specific number of signal components. This means, for example, for a code 1000 (binary) that exactly one response signal component is present and for the code 1111 (binary) exactly 4 response signal components. A short pulse is used as the interrogation signal.
In the processes described in the X-Cyte patents, the SAW element modifies additionally the phases of the response signals. The number of response signal components is thereby independent of the realized code; this is an advantage for decoding. The interrogation signal is a so-called chirp signal whose frequency varies in serrations in the range of 905-925 MHz. The SAW element comprises 16 different propagation paths (acoustic encoding channels). As a result, there are 16 different response signals whose signal periods are determined for all SAW elements of the system such that they each differ by one predetermined time interval xcex94T. The response signals expanding in the different paths thus have a time cascading which is constant (i.e. equal for all tags). When mixing the interrogation signal with the response signals, a predetermined number of known difference frequencies is generated in the interrogation station.
The difference frequency signals correspond to the beats between the interrogation signals and the time-delayed response signals. They are processed by correspondingly tuned filters. Since in each propagation path of the SAW element attenuation or phase shift elements are incorporated corresponding to the transponder-specific code, the transponder-specific code information can be obtained from the phases or amplitudes of the difference frequency signals.
Temperature changes and production tolerances cause disturbing propagation variations of the response signals. As a result, phase variations occur which distort a decoding or make it more difficult or even impossible. Thus, a calibration process as described in the above-mentioned U.S. Pat. No. 4,734,698 is used in practice. In this process two decoding channels in the transponder must have a uniform, transponder-unspecific code. The difference between the two corresponding response signals is used as the reference for the more exact phase information determination of the other response signals.
The process described above or similar processes have some of the following problems. At a frequency of 2.45 GHz, the phase encoding used is very susceptible to disturbances since already little temperature variations lead to great phase changes. If, for example, lithium niobate (0.7%/100xc2x0 C.) is used as the substrate material for the SAW element, at a temperature change of 100xc2x0 C. and a relative propagation difference of 100 ns, the relative phase change of the corresponding response signals is about 230xc2x0 at 905 MHz and about 615xc2x0 at 2.45 GHz. At 2.45 GHz ambiguity problems arise, or the reflectors must be arranged so close together that resolution problems arise and/or problems concerning the positioning of the phase elements used for encoding.
A further problem results from the described kind of calibration. The use of two response signals having a uniform, transponder-unspecific encoding reduces the number of the response signals usable for identification by 2. In the described embodiment of X-Cyte, the number of independent codes is thus reduced by the factor 4xc3x974=16 as compared to the same system without calibration.
A further problem results from the necessity to optimally separate the response signals and disturbing signals. The disturbing signals are generated outside the transponder (e.g. by reflections of the response signal at metallic objects) as well as inside the transponder (e.g. multiple reflections between transducer (converter) and reflectors in the acoustic channels). It is known that the disturbing internal reflections can be reduced effectively if in the response signal the component having the longest propagation is at least twice as long as the shortest propagation. However, in the known realization of the SAW element and in case there is a large number (e.g.  greater than 6) of encoding channels, this prerequisite leads to SAW elements requiring large chip surfaces. A further problem arises from the demand for a cost-saving production of the SAW element. In this connection, the size of the SAW element is an important factor for the cost per article. The smaller this size, the cheaper is the tag. In this connection, the known SAW tags are not satisfactory.
Further problems arise in the process for encoding the SAW elements mentioned in U.S. Pat. No. 4,096,477 in that the number of the response signals to be processed is code-specific, the internal disturbing signals (multiple reflections between the transducers) are strong and numerous and a large number of output transducers is required (e.g. 216 codes require 16 output transducers).
A further problem is to convert the incident interrogation signals most efficiently in response signals, i.e. with minimum losses. The better the interrogation signals are reflected by the reflectors, the greater is e.g. the maximum reading distance that can be achieved. Suitable reflectors for this process are e.g. described in U.S. Pat. No. 4,737,790. They operate with a basic frequency of about 915 MHz. The manufacturing of reflectors with a basic frequency of about 2.45 GHz is very difficult with respect to production technology since the width of the electrode fingers is about 0.4 xcexcm and thus leads to considerable cost disadvantages. The use of reflectors operating on the third harmonic and having a electrode finger width of about 0.6 xcexcm is known from the prior art e.g. K. Yamanouchi, G. Shimuzu and K. Morishita, xe2x80x9c2.5 GHz-range SAW propagation and reflection characteristics and application of passive electronic tag and matched filterxe2x80x9d, Proc. IEEE Ultrasonics Symposium 1993, pp 1267-1271. The exact width of the electrode fingers particularly depends on the substrate material used. The above indications relate to 128xc2x0-LiNbO3.
A further problem results from maximizing the number of possible codes (cost reduction) and, at the same time, the strength of the response signals (long reading distance or safe reading-out in an environment with strong disturbance signals) on a given chip surface.
An effective increase in the response signal strength is achieved if as few acoustic channels as possible are to be realized. As stated in the prior art (V. P. Plessky et al., xe2x80x9cSAW Tags: New Ideasxe2x80x9d, 1995 IEEE Ultrasonics Symposium), an additional loss of 12 dB results if instead of a transducer having 2 acoustic channels a solution with 4 transducers and 8 acoustic channels is realized.
In order to generate a plurality of different codes with a few or even only one acoustic channel, it is advantageous to put several reflectors in the same channel, as can be taken from the prior art (e.g. L. Reindl et al., xe2x80x9cProgrammable reflectors for Saw-ID-tagsxe2x80x9d, 1993 IEEE Ultrasonic Symposium). If more than one reflector are located in a channel, multiple reflections between the reflectors occur, which multiple reflections may be disturbing particularly if the described position encoding is used. The disturbing influence can be reduced if, for example, for a plurality of reflectors a small reflectivity is selected or if the reflectors are placed at a great distance of each other. Both possibilities lead to a weakening of the response signals of the corresponding reflectors; this is disadvantageous for the identification system and reduces, for example, the maximum reading distance.
It is an object of the invention to provide a process of the above-mentioned kind which allows an identification which is easily realizable and hardly susceptible to disturbances and, additionally, allows a great encoding variety (i.e. a large encoding space) and a good utilization of the chip surface of the SAW element.
The solution according to the invention is defined by the features of claim 1. By combining the position encoding with a calibration, identification can most reliably be performed in particular in case of a large temperature range and a small chip surface. Calibration can in principle be replaced by any kind of measurement (in particular temperature measurement). Even the parallel use of a calibration system and a measuring system is possible.
The interrogation station preferably uses a chirp signal for transponder interrogation. The response signal can be stored as a sampled, digitized time signal and can undergo a discrete Fourier transformation (FFT) for decoding the identifying information. In the frequency domain the propagation differences may be determined and processed more easily. With the realization of the SAW elements according to the invention, an effective elimination of internal disturbing signals and a cost-saving realization of the transponders is achieved.
Encoding leads to a characteristic time delay of at least part of the response signal components. It is an important feature of the process that the number of identifying signal components is independent of the implemented code word. It is identical for all transponders of a specific application. The known matched filter technique is suitable for the determination.
Exactly one response signal component (calibrating response signal) is used to measure the disturbing propagation variations of the basic delay TO and to carry out the correction of the propagations of at least the identifying response signal components. It is an important feature of the new process that the temperature-dependent relative propagation variations among the identifying response signals can be neglected.
The propagations of the identifying response signals are determined such that each signal lies in exactly one predetermined time window (designated A,B,C,D, . . . ). The sum of all time windows is a chain of non-intersecting time intervals. Each time window (enumerated 0,1,2,3, . . . ) is subdivided into predefined time slots known to the system. According to the invention the propagations of the identifying response signals are preset such that the signal, or the center of energy mass of the signal, is located in one of the predefined time slots (designated e.g. A0, i.e. in time slot 0 of time window A). The entirety of used time slots forms a characteristic pattern on the time axis (or in the spectrum), said pattern corresponding to the code number to be read out.
The size of the used time slots depends on the bandwidth of the system, in particular of the interrogation signal. In a preferred embodiment the bandwidth of the system is 40 MHz and the size of the time slots is at least 25 ns. The number of time windows NZF (e.g. 8) is predetermined by the number of encoding channels or the identifying response signals, respectively. If the number of time slots per time window is NZS (e.g. 4), the following number N of possible codes results:
N=NZSNZF, (e.g. 48=216)xe2x80x83xe2x80x83(1) 
It is an important advantage of the present invention that a large number of codes can be generated with few transducers and reflectors. In case two encoding channels with one reflector each are used, a total of one transducer and two reflectors is required to realize 216 possible codes, if 16 time slots are provided for any one of the two time windows. In known processes (cf. the above-mentioned patents of the company X-Cyte) four transducers and eight reflectors are required for the same number of codes.
For calibrating, a uniform encoding (family code) for all transponders of one application is preset in a coding channel or a separate channel. Thus, all calibrating response signals of one application lie within the same time slot of the same time window. This position is known to the system and can be changed well-defined for each application, e.g. by shifting the calibrating response signal into a different time slot of the same time window. Thus, transponder families having the same code number but a different family code are defined.
Determination of the actual position of the calibrating response signal component in the spectrum of the response signal allows, in combination with the knowledge of the desired position, the approximate determination of the disturbing change of the basic delay TO. These propagation variations are caused by temperature changes in the transponder, by the changing air gap between the transponder and the antenna of the interrogation station, by the varying length of the antenna cable and by aging or temperature changes in the electrical lines between antenna and processing unit of the interrogation station.
Contrary to the calibration known from the prior art, the process according to the invention uses exactly one response signal component for calibration. This response signal component is preferably that component having the shortest propagation. It is separated from the identifying response signal components by an off-time slot having a predetermined duration.
For carrying out the calibration, preferably
the sampled and stored response signal (time signal) is weighted with a window function and subsequently undergoes a discrete Fourier transformation and
in the resulting spectrum an actual position of the calibrating response signal component is determined and compared with a preset desired position.
The deviation between actual position and desired position leads to a frequency shift xcex94xcfx89 which can be used for calibrating the time signal.
The time signal is multiplied by a function f(t)=exe2x88x92jxcex94xcfx89t, which leads to a shift of the spectrum into a position adapted to the FFT raster, and stored as a calibrated signal.
The multiplication result is weighted with a suitable window function and undergoes a new discrete Fourier transformation.
In the received spectrum the maxima of the samples are determined in a plurality of or in all frequency windows. Subsequently, it is tested whether the received maximum samples have a sufficient signal-to-noise ratio. If this is the case, the corresponding code is determined. If this is not the case, evaluation is stopped.
In order to enable a reliable determination of the code even in case of long codes and strong temperature influences, the response signal may be divided into a plurality of blocks. These blocks are processed successively, wherein in each block a temperature-dependent propagation or frequency shift is compensated and the corresponding identifying response signal components are evaluated.
In order to further minimize temperature influences and the like when determining the code contained in the identifying response signal components, an additional, merely calculated correction may take place. For this, a temperature-dependent frequency shift xcex4xcex94xcfx89 is determined from the spectrum of the calibrated time signal by correlating the positions of all samples being sufficiently close to the calibrating sample with their desired position. (xe2x80x9cSufficiently close toxe2x80x9d means that the maximum disturbing influences to be expected can by no means cause an essential shift of the samples taken into consideration in this first block.) The stored and calibrated time signal is subsequently multiplied by a function f(t)=exe2x88x92jxcex4xcex94xcfx89t, weighted with a window function and Fourier transformed. In this way the resulting spectrum is processed successively, wherein, however, in a suitable number of not yet decoded frequency windows the maximum samples are determined and it is tested whether they have a sufficient signal-to-noise ratio, before the corresponding part of the code is determined.
In a further embodiment of the invention a transponder is realized such that in addition to identifying and calibrating response signal components also measuring response signal components are transmitted to the interrogation station. Thus, individually addressed measuring probes and measuring probes which are read out in a non-contact manner can be realized. Particularly the effect of temperature dependency of the signal delay, which is disturbing for the identification, can purposefully be used for temperature measurement. The calibrating response signals can thereby have two functions and are used for calibrating identifying and measuring response signals.
For performing such a measurement, the SAW element generates at least two response signal components for temperature measurement at the place of the transponder. Since the SAW element (which is in this case additionally used as a sensor) can be identified by its code, its individual characteristic values can be stored in the interrogation station. The interrogation station can thus store characteristic values for a plurality of such sensor elements so that a complete evaluation of the incoming response signals is always possible (e.g. determination of the absolute temperature instead of the temperature change only).
In a further embodiment of the invention a transponder is realized such that the interrogation and response signals in the transponder are lead through a common, propagation-increasing signal line being connected upstream or downstream of the encoding channels. The propagation-increasing effect of the common delay line allows a good separation of the response signals from disturbing influences, in particular from disturbing environmental reflections of the interrogation signal. The at least partially common use of the delay line allows a decreasing space-requirement for the SAW element at a constant number of possible codes. Thus, particularly the delay line which, according to the prior art, is present in each individual encoding channel of a SAW tag is replaced and a common additional delay line is purposefully introduced. Thus, the encoding channels connected upstream or downstream can be brought to a minimum length without shortening the entire propagation of the interrogation or response signal in the transponder.
As a rule, common delay lines and encoding channels are formed on one single SAW chip. However, it is also possible to realize the delay line on a separate SAW chip.
Propagation-increasing signal lines different from that for the signal components suitable for calibration and/or measuring purposes can be used for the identifying signal components. It is also possible that only some of the encoding channels are connected to the common delay line.
In order to save chip surface, the calibration channel and one encoding channel can partially overlap, wherein a semitransparent reflector is provided for generating the calibrating or also the encoding response signal component.
In a particularly preferred embodiment, the reflectors are realized such that they operate on the second harmonic. The period of the reflector structure equals the wavelength xcex of the operating frequency, e.g. 2.45 GHz, and the width of the electrode fingers is about half the period, e.g. about 0.8 xcexcm at 2.45 GHz and if 128xc2x0-LiNbO3 is used as the substrate material.
The function of the reflectors is similarly good for different forms of electrical connections between the individual electrode fingers. For example, all fingers can be short-circuited (FIG. 6d) or open, or connected as pairs of two (FIG. 6e), etc. A connection can be realized on the ends of the fingers or between them, and also combinations thereof are possible.
The use of the reflectors according to the invention leads to considerable advantages as compared to the 3rd harmonic reflectors known from the prior art:
1. The width of the electrode fingers is larger, about 8/6 larger; this makes reproducible manufacturing easier;
2. The reflection coefficient of the individual electrode fingers is essentially higher;
3. A high reflectivity can be achieved with clearly fewer electrode fingers; this is an advantage in connection with surface requirement on the chip and manufacturing costs.
The use of the reflectors according to the invention also leads to considerable advantages as compared to the reflectors known from the prior art and operating on the basic frequency:
1. At 128xc2x0-LiNbO3 and an operating frequency of 2.45 GHz the fundamental reflectors require a period (pitch) of about 0.8 xcexcm and an electrode finger width of about 0.4 xcexcm. The cost-saving manufacturing of such structures is difficult and, for example, outside the specification for the i-line steppers used today. In second harmonic reflectors the required period (pitch) doubles and the width of the electrode fingers is now about 0.8 xcexcm. Manufacturing of second harmonic reflectors with today""s i-line steppers does thus not cause any problems.
2. In addition, in case of a greater period (pitch) and correspondingly larger distances, electrical breakdowns between the electrode fingers by static or pyroelectric charging effects are less probable.
The present invention describes a reflector which can be used in a transponder and can more easily be manufactured and/or has a higher reflectivity than is known from the prior art.
If reflectors are arranged successively in an acoustic channel, in particular disturbing multiple reflections between the reflectors arise. A disturbing influence results in particular in combination with the described kind of encoding of the transponders by means of position encoding. This is particularly true if the reflectors are located close to each other, e.g. closer than 1 mm, for reasons of space requirements and in order to minimize propagation losses.
The present invention describes a new type of offset reflectors which is suitable particularly for the use in transponders. The offset reflectors result in particular from known reflectors by modifying them in accordance with the invention.
In sum, the use of offset reflectors leads to the following advantages:
1. Successive offset reflectors can be located closer to each other; thus, valuable chip surface can be saved;
2. Successive offset reflectors can generate stronger response signals since they are located closer to each other and thus fewer propagation losses result and they can have a higher reflectivity, without the simple round-trip reflections exerting a disturbing influence;
3. Disturbing multiple reflections which are generated by reflections at the transducer or at the opposite reflector on the other side of the transducer can be attenuated considerably.
In a particularly preferred embodiment second harmonic reflectors are used for the realization of offset reflectors, but all types of reflectors can be modified in accordance with the present invention. If second harmonic offset reflectors are used, in particular for an operating frequency of 2.45 GHz it is advantageous for the manufacturing process that at the end of each half an additional electric connection of the electrode fingers is introduced, which overlap partially or completely in the center of the reflector.
According to the invention, a transponder layout (e.g. FIG. 6c) can be realized with offset reflectors, wherein all reflectors 50.1-7 and 51 contained in the drawing are offset reflectors. As compared to a similar layout with four transducers, eight acoustic channels and one reflector in each channel, the present embodiment comprises only two transducers, four acoustic channels and two offset reflectors in each channel. A reflector 51 is used as a calibrating reflector, wherein it creates the shortest time delay. With the same number of, for example, five time slots per time window (one of them being an off-time slot) and a total of seven time windows, 47=16384 possible codes result for the layout realized in FIG. 6c and a corresponding layout with four transducers.
If only two transducers are used, the strength of the response signals is increased by the factor 4, since when the data carrier receives the interrogation signal, the signal is distributed to two transducers only, and only two instead of four transducers are excited after reflection.
With the layout according to FIG. 6c the surface of the chips 54 can additionally be almost halved with the same number of codes by changing from four transducers with a total of eight normal reflectors to two transducers with eight offset reflectors.
According to the invention, there is a second possibility for realizing the offset reflector 51, i.e. in each of all four acoustic channels 52.1-4 one reflector (offset or not) is located at the same distance from the transducers as 51, however with clearly lower reflectivities. The advantage thereof is that a strong calibration signal is generated by all four reflectors; however, due to the lower reflectivity of the individual reflectors, response signal disturbance by the offset reflectors 50.1-7 is lower than would be the case if only one offset reflector is located in the channel 52.1. The realization of a strong calibration response signal can also be used in cases different from the mentioned one.
In practice, the arrangement according to the invention comprises a plurality of mobile transponders or SAW tags (depending on the application, for example several tens or several ten thousands). Moreover, interrogation stations can be provided on a plurality of places (depending on the respective system demands).
An arrangement for carrying out a non-contact remote interrogation according to the present invention comprises an interrogation station for emitting an interrogation signal and for receiving and evaluating a response signal, at least one mobile transponder having an antenna for receiving the interrogation signal and/or emitting the response signal and a decoding unit having a plurality of parallel encoding channels for converting the interrogation signal into an identifying response signal. According to the invention, in front of or after the at least one encoding unit a propagation-increasing signal line common to the corresponding encoding channels is provided.
If delay line and encoding unit are integrated on the same SAW element, the corresponding structures can be coupled by transducers (finger electrode structures), 90xc2x0 reflectors (e.g. coupling of two parallel channels by two 90xc2x0 reflectors aligned with respect to each other) or RMSC structures (RMSC=reversing multistrip coupler). Contrary to the embodiments comprising transducers (in which parasitic reflections of the signal can occur at xe2x88x9215 to xe2x88x9220 dB), the 90xc2x0 reflectors and the RMSC structures are to a large extent free of disturbing internal reflections.
It is not urgently necessary that the response signal components are generated by, for example, code-specifically positioned reflectors. It can also be the case that a plurality of encoding channels is provided with transducers emitting on one side, wherein transducers received on the emitting side are arranged at code-specific distances, which are connected with the antenna (via electrical lines).
Further preferred embodiments and feature combinations can be taken from the following detailed description and from the complete set of claims.